The present invention relates to a head positioning mechanism for a magnetic disc memory device.
A magnetic disc memory device contains a head positioning mechanism for positioning a magnetic head at a track on a magnetic disc to and from which magnetic data is recorded and reproduced. The prior head positioning mechanism is comprised of a carriage carrying a magnetic head, a guide rail for rectilinearly guiding the carriage, a belt partially fixed to the carriage, and a step motor for rectilinearly moving the carriage along the guide rail upon the drive of the belt. An example of the head positioning mechanism is disclosed in U.S. Pat. No. 3,946,439, in which a belt is wound around a motor pulley of a step motor and another pulley, and a magnetic head is mounted on the carriage.
The positioning mechanism of the type using the step motor encounters a hysteresis phenomenon in forward and reverse drives of the motor rotation, resulting in deterioration of the accuracy of the positioning. FIG. 1 shows a characteristic curve representing a relationship between a holding torque and a rotation angle in a one-step rotation of the step motor. In the figure, an arrow 1a directing from point A to point Ao represents a relationship between torque and angle when the motor is rotated forwardly to make the positioning of the head. An arrow 1b directing from point B to point Bo represents a relationship between torque and angle when the motor is reversely rotated to make the positioning. When the positioning is performed through the forward rotation of the motor as indicated by the arrow 1a and the holding torque is zero, the positioning is made at an angle as indicated by point Ao. Conversely, when the positioning is performed through the reverse rotation of the motor as indicated by the arrow 1b, the positioning is made at an angle as indicated by point Bo.
As described above, the positioning mechanism by the step motor has a dead zone C in the forward and reverse rotations. The dead zone is a serious problem in the positioning mechanism requiring a high positioning accuracy.
Thus, particularly, the conventional positioning mechanism using the step motor as a drive source of the magnetic disc, which requires a high accuracy positioning between the magnetic disc and the magnetic head, can not have an accuracy of more than 50 TPI (track per inch). Therefore, the positioning mechanism can not meet the requirement of high density of recording tracks for the magnetic disc.
In the conventional positioning mechanism by the step motor in a magnetic disc device, excessive external force is never applied to the step motor. It is for this reason that, even when the recording medium (disc) is replaced by another medium, it is required that the magnetic head is positioned at a given absolute position for keeping a relative position of the track to the magnetic head. Accordingly, the prior positioning mechanism normally makes the positioning in the dead zone C shown in FIG. 1. For this reason, the accuracy of the positioning depends on the dead zone, resulting in poor positioning accuracy.